The present invention relates to a light-diffusing film used in a screen for a front projector, a screen for a rear projector, or a backlight for a liquid crystal display, and a screen including the light-diffusing film.
Recently, overhead projectors and slide projectors have been widely used when a speaker presents a document in a conference or the like. Video projectors and motion picture film projectors using liquid crystal have also been widely used in households. In these projectors, light output from a light source is subjected to an optical modulation with a transmissive liquid crystal panel or the like to form image light. This image light is emitted through an optical system such as a lens and projected on a screen.
In the above-described projector units, a screen for a projector is used to view a projected image. This screen for a projector is broadly divided into a screen for a front projector and a screen for a rear projector. In the front projector, a light source emits projection light from the front side of a screen, and the projection light reflected on the screen is viewed. In the rear projector, a light source emits projection light from the back-side of a screen, and the light transmitted through the screen is viewed from the front side of the screen. Both screens used in the above methods preferably have a satisfactory visibility and a wide viewing angle.
For this purpose, a light-diffusing film for scattering light on the surface of a screen is generally provided in both methods. This light-diffusing film allows image light to be emitted by diffusing uniformly on the entire effective area of a screen.
Known light-diffusing films are broadly divided into an isotropic diffusion sheet and an anisotropic diffusion sheet. Since the amount of incident light is the same in these two sheets, the anisotropic diffusion sheet, which can improve the luminance by diffusing light only in the desired direction, has drawn attention. In particular, when a light-diffusing film is used in a projection screen, the field of view in the horizontal direction is more important than that in the vertical direction. Therefore, an anisotropic diffusion sheet having a strong diffusing capacity in the horizontal direction has been developed.
Known methods for making such an anisotropic light-diffusing film includes a method of forming a speckle pattern on a photosensitive resin, the speckle pattern being formed by irradiating a coherent beam of light on a rough surface (see, for example, Japanese Unexamined Patent Application Publication Nos. 53-51755 and 2001-100621); a method of exposing such a pattern on a photosensitive resin using a mask; and a method of directly cutting the surface of a mold base material such as a metal or a resin by machining to prepare a mold having fine irregularities thereon, and transferring the shape of the irregularities from the mold using a UV curable resin or the like (see, for example, Japanese Unexamined Patent Application Publication No. 2000-284106).
In addition, a method of applying a mixture of a resin binder and resin beads dispersed in the resin binder on a transparent substrate is known.
However, scratches are easily formed on the surface of a known anisotropic light-diffusing film. As a result, a light-diffusing capacity is lost, thus causing image defects. In a method of making an anisotropic light-diffusing film including a mixture of a resin binder and resin beads dispersed therein, the scratch resistance can be improved using soft beads (see, for example, Japanese Unexamined Patent Application Publication No. 2000-275410). However, because of scratches, the beads are detached even by this method. Furthermore, in this case, although beads having an anisotropic shape are used in the coating in order to provide anisotropy, the orientation of the beads is insufficient and a satisfactory light-diffusing anisotropy is difficult to be achieved.
In addition, the surface of the anisotropic light-diffusing film is damaged during storing or handling, thus causing image defects.
In the method of making an anisotropic light-diffusing film including transferring an irregular shape on the surface of a mold, a resin is damaged when the resin is released from the mold after curing. This damage may cause image defects.